1. Technical Field
The present invention relates to a method of decoding hierarchically orthogonal structured light and an 3-D depth measurement system using the same and, particularly, to a method of decoding hierarchically orthogonal structured light which enables precise 3-D depth measurement because a deformed signal can be precisely restored using a new scheme called signal separation coding and a 3-D depth measurement system using the same.
2. Related Art
In general, a 3-D depth measurement method using structured light is recently in the spotlight because the 3-D depth measurement method is suitable for sensing a 3-D environment in service robot engineering. A basic principle of depth measurement using structured light, that is, an active stereo scheme lies in that a ray of light is radiated to an object using a projection apparatus, such as a projector, the object to which the ray of light has been radiated is captured using an image reception apparatus, such as a camera, the depth to the object is calculated by monitoring how degree is the ray of light distorted by the object, and a depth image is obtained based on the calculated depth.
FIG. 1 is a schematic diagram for illustrating the principle of a 3-D depth measurement system based on structured light. As shown in FIG. 1, the 3-D position of one point x of the target object 100 is determined as an intersection point where a straight line, coupling the original point Op of projection means and a point p on the image plane 200 of a projection apparatus, meets a straight line that couples the original point Oc of an image reception apparatus and a point q on the image plane 300 of the image reception apparatus. Accordingly, if the projector (i.e., the projection apparatus) and the camera (i.e., the image reception apparatus) have been corrected, the depth image may be obtained by calculating the coordinates of the x point as a pair of address values in each image plane of the points p and q. That is, in this stereo scheme, the core of the method of measuring the depth image is to determine a point corresponding to a pixel in a received image and a projected image. When the corresponding point is determined, the distance may be easily calculated according to simple geometry.
For the accuracy of depth measurement, a light pattern projected from the projection apparatus is temporally coded according to spatial and/or time sequences on a pixel array so that the spatial and/or temporal addresses of a signal detected by the image reception apparatus may solely determine a point corresponding to a pixel of a corresponding projection apparatus.
As related to this application, there is Korean Patent Registration No. 10-0588296 for a 3-D image generation apparatus using orthogonally hierarchically structured light in which the pixel of each projection apparatus is addressed and projected using orthogonally hierarchically structured light and the results are decoded in an image reception apparatus. A conventional technique for hierarchically structured light, proposed in Korean Patent Registration No. 10-0588296, is simply described using FIG. 2. FIG. 2 is an exemplary diagram showing projection light of hierarchically orthogonal light, having three hierarchical layers, that is projected on a projection region. As shown in FIG. 2(a), in a layer 1, a projection region is addressed by light projected between a point of time t0 and a point of time t3. A projection region is addressed into (1 0 0 0) and projected at the point of time t0, a projection region is addressed into (0 1 0 0) and projected at the point of time t1, a projection region is addressed into (0 0 1 0) and projected at the point of time t2, and a projection region is addressed into (0 0 0 1) and projected at the point of time t3. Likewise, as shown in FIG. 2(b), in a layer 2, each the regions of the layer 1 is addressed into a detailed region and projected. The layer 2 includes a point of time t4 to a point of time t7. A projection region is addressed into (1000 1000 1000 1000) and projected at the point of time t4, a projection region is addressed into (0100 0100 0100 0100) and projected at the point of time t5, a projection region is addressed into (0010 0010 0010 0010) and projected at the point of time t6, and a projection region is addressed into (0001 0001 0001 0001) and projected at the point of time t7. Finally, in a layer 3, as shown in FIG. 2(c), each of the regions classified in the layer 2 is addressed into a detailed region and projected, and the layer 3 includes a point of time t8 to a point of time t11. A projection region is addressed into (1000100010001000 1000100010001000 1000100010001000 1000100010001000) and projected at the point of time t8, a projection region is addressed into (0100010001000100 0100010001000100 0100010001000100 0100010001000100) and projected at the point of time t9, a projection region is addressed into (0010001000100010 0010001000100010 0010001000100010 0010001000100010) at the point of time t10, and a projection region is addressed into (0001000100010001 0001000100010001 0001000100010001 0001000100010001) and projected at the point of time t11.
As shown in FIG. 2, when an image is projected on the projection apparatus, the image reception apparatus may check information about the depth of a 3-D image by decoding the address of the projection apparatus corresponding to each pixel. A system using hierarchically structured light as in FIG. 2 is problematic in that a boundary line becomes unclear in an image signal received by the image reception apparatus depending on a geometric shape of a target object placed in a relevant boundary region, the degree that light is reflected, external environments, etc. in classifying regions according to precise timing in the image reception apparatus and projecting light. However, a conventional technique had many problems because an image is classified according to specific timing when decoding addresses included in a received image and the addresses included in the received image are restored despite of the above problem.